The application of an antifouling coating composition comprising a binder resin and an antifouling agent onto the surfaces of ship's bottom or underwater constructions has been practiced for a long time.
However, in such coating compositions, since the binder resin is usually of resistant nature to the attack of sea water and the like and the antifouling effect is entirely dependent on the antifouling agent dissolved out of the coating, even though the applied coating is effective at the beginning stage when a high speed of dissolution is obtained at the surface of said coating, the rate of dissolution is lowered as it comes into the stage wherein the dissolution is originated from the diffusion phenomenon caused by the concentration gradient of the antifouling agent contained. And thus, the coating will lose its effectiveness in a comparatively shorter period of time. Furthermore, after said dissolution, the coating will form a skeleton structure, which in turn causes, in case of ship, a speed loss and an increase in fuel expense. Under the circumstances, an antifouling paint based on a hydrolysis type resin having incorporated therein an antifouling component and especially an organo tin component has become the center of public attention.
In this type of coating composition, tributyl tin groups are, in general, contained in the binder resin via ester bonds and during sailing, these toxic tributyl tin groups are gradually released from the resin through hydrolysis in a weak alkaline sea water and at the same time, the resin per se will get a number of hydrophilic groups, becomes soluble and will be gradually dissolved into sea water. Thus, the coating shows a self-polishing characteristic and hence, this type of coating composition is far superior to other heretofore proposed compositions in that most of the antifouling agent contained in the coating are effectively utilized as the toxicant.
However, to get an appropriate hydrolysis rate and an optimum polishing rate, it is usually required to provide a fairly large number of hydrolysis sites, i.e. organo tin bonding sites, in the resin. This also provides hygienic and economical problems as might be easily understood. As an attempt to decrease the amount of organo tin compound required, Japanese Patent Application Kokai No. 118868/83 suggests the inclusion of number of hydroxyl groups in the resin, but this is rather difficult to do and obtain a good result.